1. Field of the Invention
The present invention relates to the improvement of waveform measuring instruments using equivalent time sampling, such as digital oscilloscopes.
2. Description of the Prior Art
The digital oscilloscope which is a typical waveform measuring instrument converts a time-series continuous signal waveform into digital data with an A/D converter, records the data in a memory discretely, and displays the data recorded in the memory as waveforms.
In such waveform measuring instruments, there is a repeated data acquiring mode (hereafter called “repeated mode”) using equivalent time sampling as one of the methods for recording signal waveforms on the memory.
FIG. 1 is a block diagram showing a conventional embodiment in which the repeated mode function of the digital oscilloscope is considered.
Pre-amplifier 1 comprises the attenuation circuit and the pre-amplifier and adjusts the amplitude of input signals so that they come into an appropriate range for the input specifications of A/D converter (hereafter called “ADC”) 2 to output them to ADC 2.
ADC 2 converts input signals to digital data items at the timing of the clock input from time base 6. The output data items of ADC 2 are transferred to primary memory 5 through primary memory controller 4 and accumulated one by one. Although the output data items of ADC 2 are accumulated in turn in primary memory 5 as waveform data items, a trigger signal is output from trigger circuit 3 to time interval measurement circuit 7 at the instant when the input waveform meets the desired trigger conditions set by an observer.
Time interval measurement circuit 7 measures the time difference between the time base and the trigger signal and sends that time difference information to primary memory controller 4.
Primary memory controller 4 sends the data read from primary memory 5 and the time difference information to acquisition memory controller 8.
Acquisition memory controller 8 transfers the data items to corresponding time slots in acquisition memory 9 based on the time difference information given from time interval measurement circuit 7.
By this series of actions, data items for a part of the waveform are acquired. Accordingly, by repeating this series of actions two or more times, data items for the entire waveform are recorded in acquisition memory 9 so that the entire waveform is re-configured.
Waveform data processor 10 implements data processing such as addition, subtraction, multiplication, etc. among a plurality of waveforms against the data items read from acquisition memory 9, and prepares the display data items corresponding to the resolution of a display. The data items processed in waveform data processor 10 are input to display processor 11.
Display processor 11 writes the display data items into display memory 12 as well as outputs the display data items in display memory 12 to display 13 such as a LCD, CRT, printer, or the like.
FIG. 2 is a drawing illustrating the relationship between the waveform data items and acquisition memory 9. In FIG. 2, “◯” represents the sampling data items in time slot 1, “Δ” represents the sampling data items in time slot 2, and “×” represents the sampling data items in time slot 3. Box A in FIG. 2 expresses the configuration of acquisition memory 9 in a time series arrangement, corresponding to addresses. A set of addresses having the same symbol constitutes a time slot. In addition, Te represents the interval of equivalent time sampling data items formed by a time series arrangement of real time sampling data items in each time slot, and Tr represents the period of real time sampling data items based on actual sampling timing in each time slot.
FIG. 3 is a drawing illustrating an example of mapping of acquisition memory 9. The inside of acquisition memory 9 is divided into regions based on the concept of time slots and an address group in a predetermined region in the memory is mapped for each one of the time slots. In other words, the sampled data items for a part of a waveform obtained by one-time data acquisition are recorded in an area referenced by each address within a time slot. Since the minimum value of time intervals between time slots is equal to the resolution of time interval measurement circuit 7, the maximum number of time slots becomes dependent on the resolution of time interval measurement circuit 7.
In the meantime, there is a case where signals on the data bus, so-called “eye pattern”, are observed as one of the waveform observations using the repeated mode of the digital oscilloscope. A feature of this waveform is that a plurality of voltage values exists at an instant although it is a repeated waveform.
However, in digital oscilloscopes having a conventional configuration as shown in FIG. 1, there is a problem that, if different voltage values are sampled for the same time slot, the prior measured data item is overwritten by the later measured data item and thus the prior measured data item is not recorded because only one data item can be recorded at an instant.
In addition, if zoomed waveforms of such signals are displayed, intervals between displayed data items are extended and thus the number of data item points that can be displayed on a waveform displaying screen decreases because of the limitation in resolution of the time interval measurement circuit, and this may possibly cause those waveforms not to be recognized as a time series continuous waveform group. FIG. 4 shows an example of such waveforms. In FIG. 4, symbols “◯” shown with a broken line represent the points that have disappeared from the memory by overwriting though sampled voltage values exist. If such a situation occurs, reproduction and display of the original waveforms may be almost impossible.